1. Field of the Invention
The present invention relates to a power transmission device for sewing machines, and in particular, to a power transmission device for transmitting rotation of a drive shaft or for transmitting rotation of a main shaft to a slave shaft that extends within a plane parallel to the plane of the drive shaft, but is not parallel to the drive shaft. Such a relationship between the drive shaft and the slave shaft is known as a xe2x80x9ctorsional relationship.xe2x80x9d
2. Description of the Related Art
A known sewing machine includes a plurality of sewing heads each having an upper shaft for driving a sewing needle and a thread take-up lever, and a lower shaft for driving a shuttle. The lower shaft normally extends in parallel with the upper shaft. The upper shaft and the corresponding lower shaft are coupled to each other by means of pulleys and a timing belt, so that the upper and lower shafts can be driven in synchronism with each other. A single main shaft extends through a plurality of gear boxes that correspond to the plurality of the sewing heads at a right angle relative to the upper and lower shafts. The main shaft is driven by a drive source, such as a motor. A plurality of bevel gears are mounted on the main shaft at a position within the respective gear boxes. A bevel gear is mounted on one end of the upper shaft and is positioned within the corresponding gear box to engage one of the bevel gears of the main shaft. As a result, when the drive shaft rotates, the rotation of the drive shaft is transmitted to the upper shaft of each sewing head and is then transmitted to the lower shaft by means of the timing belt.
The drive shaft and each of the upper shafts or slave shafts in the known sewing machine are positioned in a torsional relationship with each other. Therefore, the bevel gears are used as a power transmission device in the known sewing machine.
However, because of the incorporation of the bevel gears in the known sewing machine, significant vibrations and noise are produced from each pair of engaging bevel gears, in particular when the drive shaft rotates at high speed. Therefore, the working environment may become uncomfortable and noisy for the operator.
It is, accordingly, one object of the present invention to teach improved power transmission devices for a sewing machine, in which the rotation of a drive shaft can be transmitted to a slave shaft without producing substantial vibrations or noise.
According to the present invention, improved power transmission devices are taught that transmits rotation of a main shaft or a drive shaft of a sewing machine to a slave shaft of the sewing machine. The slave shaft extends in a torsional relationship with the drive shaft. The slave shaft may serve to drive a needle bar and a thread take-up lever or may serve to drive a shuttle. The power transmission device may include a first timing pulley mounted on the drive shaft, a second timing pulley mounted on the slave shaft, and a timing belt extending between the drive shaft and the slave shaft in a twisted form.
Therefore, the rotation of the drive shaft may be transmitted to the slave shaft by means of the timing belt, so that vibrations and/or noise may be substantially reduced in comparison with known sewing machines that utilize gears as a power transmission device.
In a preferred embodiment, the positional relationship between the first timing pulley and the second timing pulley is determined such that the timing belt will not shift in a sideways direction when the rotation of the drive shaft is transmitted to the slave shaft.
Such a positional relationship may be achieved, for example, by positioning the rotational axis of the first timing pulley perpendicular to the rotating axis of the second timing pulley. Further, the lateral surface of the timing belt at the position in which the leading portion of the timing belt departs or separates from the first timing pulley is aligned with one peripheral edge of the timing belt at a position where the timing belt first contacts the second pulley as shown in FIG. 3.
Therefore, damage to the timing belt may be substantially reduced even during the extended operation of the sewing machine.
In another preferred embodiment, the transmission device further includes a second slave shaft that may extend parallel to the first slave shaft. A second timing belt may be provided to transmit rotation of the first slave shaft to the second slave shaft. Therefore, one of the first and second slave shafts may serve to drive the needle bar and the thread take-up lever, and the other of the first and second slave shafts may serve to drive the shuttle.
Other objects, features and advantages of the present invention will be readily understood after reading the following detailed description together with the accompanying drawings and the claims.
FIG. 1 is a sectional side view of one of sewing heads of a representative sewing machine having an improved power transmission device;
FIG. 2 is an enlarged view of the power transmission device;
FIG. 3 is a right side view of FIG. 2;
FIG. 4 is a view similar to FIG. 2 but showing a preferred positional relationship of the timing pulleys and the timing belt;
FIG. 5 is a right side view of FIG. 4; and
FIG. 6 is a sectional side view of one of the sewing heads of a second representative sewing machine.